Orthogonal frequency division multiplexing (OFDM) has become the physical layer of choice for many wireless communications systems. An attractive feature of current wireless local area network (WLAN) and wireless metropolitan area network (WMAN) standards based on OFDM is the designed ability to operate in unlicensed spectrum. However, these systems must share spectrum with other unlicensed systems; such as cordless telephones, garage door openers, baby monitors and microwave ovens; which produce narrowband interference in WLAN and WMAN systems. Further, radio non-idealities such as transmitter carrier feedthrough (also known as carrier leakage) also introduce narrowband interference in the form of single-frequency carrier residues.
Pilot symbol assisted systems are particularly susceptible to narrowband interference during receiver detection and synchronisation. Pilot symbol assisted systems are also susceptible to narrowband interference on the data transport phase of receiver operation. An interference suppression technique has been proposed to improve the performance of pilot symbol assisted detection and synchronisation in the presence of narrowband interference, see PCT/NZ2004/000060. However, this technique cannot be applied during data transport as it introduces inter-symbol interference.
Previous proposed interference suppression systems for OFDM include using pre-coding, spread spectrum OFDM, and post-detection receiver techniques involving equalizers. There are many literature reports on narrowband interference suppression techniques for spread spectrum systems, including excision-based methods. However the interference suppression requirements for OFDM differ significantly from the requirements for spread spectrum.
Previously proposed methods for narrowband interference suppression include using pre-coding and spread spectrum techniques. These methods require modifications to the transmitted OFDM signal which are not supported by current OFDM standards. Frequency domain techniques also have been proposed, but these either do not take account of spectral leakage from the interference, or require substantial processing to estimate and remove the interference from all OFDM frequency bins. Additionally, another proposed method requires co-operation of the transmitter in not sending data on a number of sub-carriers in the vicinity of (that is, close to in frequency) each narrowband interferer. Again, this ability to modify the transmitted signal is not supported by the current standards.